The invention relates to a method and apparatus for tracking process and product data between various measurement and control stations on a processing line. The application discussed is a continuous galvanizing (zinc coating) line; however, the system can be generally applied to any strip processing line, including aluminum coating lines, tin coating lines, paint coating lines, etc., with measurement and control stations spatially separated. The system provides a means for continuously evaluating a process-controller action at a previous control station by tracking pertinent processing data to a measurement station where it can be used with the measurement data to determine further control action. The system also provides a means for ideal process setup by tracking information, such as where the beginning product of a new order is throughout the line, so that new set points can be applied to process controllers at exactly the correct time. For example, a typical objective of this invention might be to follow the progress of welds (new orders) through the galvanizing line, to follow processing data from the zinc pot to an in-line coating gauge, and to follow coating data, from the gauge, through the exit loop and then to the shear. This system is particularly applicable on processing lines with several discrete sections provided with variable storage capacity between sections to allow for independent control of the strip speed in each section.
Weld tracking systems that have been used in the past require that a hole be punched in or a magnetic mark be placed on the strip when a weld is made. The arrival of this hole or magnetic mark at a specific reference point along the line is then detected by special detection equipment permanently mounted in line at the reference point location. Because of this need for permanent mounting of the detection equipment, these systems are very limited in that, for economical reasons, only a small number of reference points can be accommodated. Systems of this type are highly undesirable in that the hole punching (or magnetic marking) equipment and the special detection equipment are generally unreliable and require excessive maintenance. The tracking system of this invention provides an extremely reliable and versatile weld tracking system and, in addition, provides the capability for tracking processing data through a processing line for purposes of automatic control of the process and documentation of critical processing parameters.
In a typical coating operation of the type to which this invention is directed, the strip material such as steel, aluminum, paper, etc., is moved from an entry station, which includes a weld machine for welding front ends of a new coiled strip to the rear end of a previous strip, through cleaning tanks, and then to a storage facility consisting of an entry loop section. The simplest type of entry loop consists basically of a single loop of the strip which falls into a loop pit which may typically be 50 feet deep. Alternatively, the storage loop may include sets of stationary rollers and corresponding sets of movable rollers mounted on a "loop car." The loop car is motor driven to allow the loop to be shortened or lengthened as appropriate. From the loop storage facility the strip may proceed through an annealing furnace to the coating bath, such as a molten zinc bath having a nominal temperature of about 900.degree. F.
From the coating bath the coated strip travels upwardly in free travel a distance sufficient to cool the coating material. During this travel, the coated strip is moved past a pair of air knives located one on each side of the moving strip. These air knives force air under pressure against the coated surface to adjust the coating thickness by wiping excess coating material downward toward the bath. The air stream is typically directed at a downward angle relative to the strip and the adjustment of this angle is generally made according to the operator's preference. The air knives are also capable of movement toward and away from the strip and the pressure of the air stream is also adjustable. It has been found that the system operates most effectively by maintaining equal or near equal pressures on both sides of the strip; therefore any adjustments to the knife pressure are made equally to both knives.
From the coating station, the strip travels to a measuring station which may be several hundred feet from the air knife location; this is the closest point to the coating bath that provides an environment in which the measuring apparatus can operate effectively.
A suitable tracking system must be used to record and track such information as the coating-weight set points and pressure correction data from the air knives to the coating gauge to compensate for the dead time between any change in operating conditions at the air knives and the detection of this change by the coating gauge. Such a tracking system is described and claimed herein.
In the tracking system, coil welds are tracked on a per-foot basis from the entry welder to the air-knife position. Product data is tracked from the air-knife position to the exit shear along with weld position. The in-line tracking equipment includes three pulse tachometers located on drive bridle motors and track switches in the looper-car or storage loop area. Signals generated by these in-line devices are combined in digital logic circuits of either the hard-wired or the programmable type to effect the tracking system functions described above. Tracking accuracy is automatically recalibrated on a regular basis by looper-car or strip loop movements which cause track-switch operation at defined distance points.